1. Field of the Invention
The present invention relates to a semiconductor laser device using a nitride semiconductor, and an optical information reproduction apparatus including such a semiconductor laser device. In particular, the present invention relates to a nitride semiconductor laser device having a low operating voltage and which generates a light spot having an ellipticity close to 1, and an optical information reproduction apparatus including such a nitride semiconductor laser device.
2. Description of the Related Art
Semiconductor laser devices for generating light in wavelength regions of blue to purple light have been produced for tests using nitride semiconductor materials represented by, for example, GaN, InN, AlN and mixtures thereof. Such semiconductor laser devices are reported in, for example, Shuji NAKAMURA et al., Appl. Phys. Lett. Vol. 69 (1996), pp. 4056-4058.
FIG. 17 is a partial schematic cross-sectional view illustrating a structure of a conventional nitride semiconductor laser device 1700 oscillating at a wavelength of 410 nm. The semiconductor laser device 1700 includes an n-GaN layer 171 (thickness: 3 μm), an n-In0.05Ga0.95N buffer layer 172 (thickness: 30 nm), an n-Al0.05Ga0.95N cladding layer 173 (thickness: 0.5 μm), an n-GaN optical waveguide layer 174 (thickness: 0.1 μm), an In0.2Ga0.8N/n-In0.05Ga0.95N triple quantum well active layer 175 (thickness: 4 nm/8 nm×3MQW), a p-Al0.2Ga0.8N layer 176 (thickness: 20 nm), a p-GaN optical waveguide layer 177 (thickness: 0.1 μm), a p-Al0.05Ga0.95N cladding layer 178 (thickness: 0.5 μm), and a p-GaN contact layer 179 (thickness: 0.2 μm). These layers are sequentially formed. A stripe ridge structure having a width of 2 μm is provided by partially etching the contact layer 179 and the p-cladding layer 178. Etching is stopped at a level in the p-cladding layer 178. An insulating layer 180 is provided so as to cover the exposed flat surfaces of the p-cladding layer 178 and side surfaces of the stripe ridge structure. An electrode 181 is provided on the insulating layer 180 and the contact layer 179. The semiconductor laser device 1700 has a waveguide structure in which the active layer 175 and the optical waveguide layers 174 and 177 are interposed between the cladding layers 173 and 178. Light generated in the active layer 175 is confined in the waveguide structure, and thus laser oscillation is caused.
The above-described conventional semiconductor laser device 1700 has the following problems.
When the present inventors produced semiconductor laser devices having the above-described structure, the resultant FFP(far field pattern) had an elliptical shape in which the full angle of half maximum (FAHM) in the perpendicular direction Θ⊥ was 18 degrees, the full angle of half maximum in the horizontal direction Θ∥ was 8 degrees, and the ellipticity (ratio between the full angle of half maximum in the perpendicular direction and the full angle of half maximum in the horizontal direction) was equal to or greater than 2. Therefore, the above-mentioned conventional semiconductor laser device has problems of, for example, (i) requiring a higher operating power due to the reduction in the power density and (ii) being unable to read information which is recorded at high density when the semiconductor laser device is used in an optical information reproduction apparatus.
An optical information reproduction apparatus usually includes two triangular prisms as shown in FIG. 18 for converting the elliptical spot of a light beam into a circular spot. FIG. 18 schematically shows a structure of a conventional optical information reproduction apparatus. It should be noted that FIG. 18 excludes parts which are not necessary for this explanation for the sake of simplicity. As shown in FIG. 18, the conventional optical information reproduction apparatus includes a base table 1001, a conventional semiconductor laser device 1002 set on the base table 1001, a coupling lens 1003, a shaping prism 1004, a beam splitter 1005, a disc 1006, and a photodetector 1007. In this apparatus, the shaping prism 1004 converts the elliptical spot of a light beam into a circular spot. The optical information reproduction apparatus including such a semiconductor laser device is inevitably large in scale and high in production cost.
This means that it is necessary to improve the ellipticity without increasing the operating power of the semiconductor laser device. However, it is not easy to separately control the operating power and the full angle of half maximum for the FFP, for the following reasons.
One technique for improving the ellipticity is to reduce the full angle of half maximum in the perpendicular direction Θ⊥. In order to realize this, light confinement in the perpendicular direction needs to be weakened, which can be realized by, most effectively, increasing the Al content of the p- or n-cladding layer to reduce the refractive index. However, when light confinement in the perpendicular direction is weakened, the threshold current for causing laser oscillation is increased, and thus the operating power of the semiconductor laser device is increased.
Another technique for improving the ellipticity is to increase the full angle of half maximum in the horizontal direction Θ∥. In order to realize this, light confinement in the horizontal direction needs to be strengthened, which can be realized by increasing the height of the ridge or reducing the refractive indices of the materials used for the buried layers. However, when light confinement in the horizontal direction is excessively strengthened, the semiconductor laser device suffers from another problem in that a high order lateral mode is likely to be caused. In other words, the degree of light confinement in the horizontal direction is naturally determined based on the contradictory relationship of sufficient suppression of the high order lateral mode and the sufficient strength of light confinement in the horizontal direction. Therefore, the full angle of half maximum in the horizontal direction Θ∥ can only be increased to an upper limit, which is in the range of 6 degrees to 10 degrees.
As described above, the conventional semiconductor laser device cannot allow the full angle of half maximum for the FFP to be improved without increasing the operating power, merely by changing the refractive indices of the nitride semiconductor materials or thicknesses of the layers. Therefore, an optical information reproduction apparatus including the conventional semiconductor laser device is limited in size reduction and power savings.